1. Field of the Invention
The invention relates generally to roller cone bits and methods for simulating such bits.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a conventional drilling system drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is a roller cone-type drill bit 20, shown in further detail in FIG. 1.
As shown in FIG. 2, roller cone bits 20 typically comprise a bit body 22, having an externally threaded connection at one end 24, and a plurality of roller cones 26 (in this case three) attached to the other end of the bit and able to rotate with respect to the bit body 22. Attached to the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of the cones 26. The cutting elements 28 can be tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
The bit body includes one or more legs, each having thereon a bearing journal. The most commonly used types of roller cone drill bits include thee such legs and bearing journals. A roller cone is rotatably mounted to each bearing journal. During drilling, the roller cones rotate about the respective journals while the bit is being rotated. The roller cones include a number of cutting elements, which may be press fit inserts made from tungsten carbide and other materials, or may be milled steel teeth.
The cutting elements engage the formation in a combination of crushing, gouging, and scraping or shearing action which removes small segments of the formation being drilled. The inserts on a cone of a three-cone bit are generally classified as inner-row insert and gage-row inserts. Inner row inserts engage the bore hole bottom, but not the well bore wall. Gage-row inserts engage the well bore wall and sometimes a small outer ring portion of the bore hole bottom. The direction of motion of inserts engaging the rock on a two or three-cone bit is generally in one direction or a very small limited range of directions, i.e., 10 degrees or less.
When a roller cone bit is used to drill in earth formation, the cutting elements, cones, and bit may experience stress. Stress occurs because of the forces applied to the bit in drilling. The amount of stress felt by any given cutting element, cone or the entire bit will depend on the amount of force applied and the surface area of the bit receiving the force. The stress experienced by a cutting element, cone or bit in drilling can be classified into two main categories: tensile stress and compressible stress. The classification into these categories depends on the direction of the forces in relation to the bit. Tensile stress leads to expansion of the bit material, while compressive stress results in compaction of the bit material.
A material can withstand a certain level of tensile stress and compressive stress before it reaches the tensile strength and compressive strength of the material. When the compressive strength is reached, the material fails by compression. When the tensile strength is reached, the material fails breakage. As a practical matter, during drilling of an earth formation, the cutting elements, as well as other parts of the bit are under tensile and compressive stresses.
One significant factor to be considered in the design of the a roller cone bit is the compressive and tensile strengths of the various components of the bit. Components made of a material with a lower tensile strength are preferably not subjected to high tensile stresses. Similarly components made of a lower compressive strength material are preferably not subjected to high compressive stresses. The amount of compressive and tensile stresses impacted on a cutting element will depend in part on the position of such particular cutting element, the position of its row and its cone. Additionally, the cone geometry, as well as the journal angle, which is the angle between the line perpendicular to the axis of the bit and the axis of the bit leg journal, will affect the amount of tensile and compressive stresses induced on a cutting, cone, and bit. By adjusting the cone geometry and journal angle, the induced stresses may vary.
Significant expense is involved in the design and manufacture of drill bits. Therefore, having accurate models for simulating and analyzing the drilling characteristics of bits can greatly reduce the cost associated with manufacturing drill bits for testing and analysis purposes. For this reason, several models have been developed and employed for the analysis and design of 2, 3, and 4 roller cone bits. See, for example, U.S. Pat. Nos. 6,213,225, 6,095,262, 6,412,577, and 6,401,839.
While the prior art methods allow for simulation of drill bit performance, where is still a need for methods to simulate and optimize the tensile and compressive stresses induced on roller cone bits drilling earth formations.